Description of the Prior Art
Heretofore, for the preparation of molded proucts of complicated profile, it is known to use, for example, an injection molding method by using as starting material metallic powders such as powders of 2% Ni -98% Fe, powders of SUS 316 or stellite powders, ceramic powders such as alumina, silicium carbide, silicium nitride or zirconia powders or metal-ceramic powder mixtures such as a mixture of ceramics and tungstene carbide or cobalt.
For example, the injection molding method is used by Parmatech Corp. for the preparation of small-sized machine components formed of 2% Ni -98% Fe or stainless steel; by the Carborundum Company for the preparation of turbine components of silicium carbide; and by Messrs. ASEA for the preparation of gas turbine wheels of silicium nitride.
In general, injection molding is composed of the following steps:
(i) the step of mixing powders of starting materials with an organic dispersive medium or binder to give a thermoplastic mixture; PA1 (ii) the step of softening the mixture in a heated cylinder and injecting the thus-softened mixture into a metallic mold; PA1 (iii) the step of opening the metallic mold to take out the molded product; PA1 (iv) the step of degreasing for eliminating the organic dispersive medium from the molded product; and PA1 (v) the step of calcination for elevating the density of the molded product.
The success of injection molding consisting of these respective steps depends notably on the kind of the organic binder employed in the process. Above all, the yield of the ultimate products is occasionally influenced by whether or not the appropriate organic binder is employed.
The purpose of using the organic dispersive medium or binder is to impart plasticity and moldability of the starting pulverulent material. In case of poor moldability, flaws such as silver marks, weld lines or sink marks are produced in the molded products.
While moldability can be improved by increasing the addition amount of the organic dispersive medium, a large amount of the binder is naturally removed in the degreasing step with the result that flaws such as crevices, deformation or foaming are likely to be induced in the molded products.
In practicing the above described molding process, it has been tried to reduce the amount of the organic dispersive medium to a smallest value possible while using such organic binder as will not cause the aforementioned defects during the degreasing step. In general, a mixture of low molecular weight polyethylene, polystyrene, paraffin or fine crystal wax and a minor amount of oil or thermoplastic resin is used as such binder.
In addition, polypropyrene, polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, methyl cellulose, atactic polyester cellulose, hydroxy ethyl cellulose or acrylic resin can also be used as the binder. It has also been customary to add a small amount of stearates into the organic dispersive medium for improving mold release properties of the molded products.
In the meanwhile, the following difficulties are presented with these conventional binders.
(i) In the above method, the organic dispersive medium is cracked and vaporized off in the degreasing step by heating to a temperature of ca. 400.degree. to 500.degree. C. At this time, it is necessary that the molded product be gradually heated to a maximum temperature so that the gas will be evolved with cracking of the binder at a rate not higher than the rate of gas diffusion by way of pores in the molded product. When the rate of gas evolution is higher than that of gas diffusion, the pressure in the molded product is increased thus occasionally causing deformation, cracking or foaming. Therefore, a prolonged processing time of 70 to 100 hours is usually required in the degreasing step so that the merit of higher productive efficiency proper to the techniques of injection molding is ultimately lost.
(ii) While the heating over an extended period of time is required in the degreasing step, the evolved heat is as low as 400.degree. to 500.degree. C. as noted above and hence economically difficult to recuperate as effective heat energy. Thus it is discarded as waste heat.
(iii) Although degreasing is terminated with completion of heat cracking of the binder, it is not possible to remove the grease completely so that small amounts of carbon or oil and fat are left in the binder. These residual amounts of carbon or oil and fat are likely to detract from the properties of the calcined product.
(iv) Since the products recovered from the degreasing process are decomposition products of the organic dispersive medium and cannot be reused, they are usually discarded, with the result that production costs are correspondingly elevated.
The aforementioned problems yet to be solved are presented not only in the techniques of injection molding but in the techniques of molding of plastic materials in general, such as those of extrusion molding or die casting.
There is also known a casting-molding method for molding of metallic powders, ceramic powders, or a mixture of metallic and ceramic powders. According to this method, a viscous slime of starting powders is cast into a liquid-absorbing mold to form a wall layer to produce the molded product.
For producing the viscous slime of starting powders, the starting powders are mixed with a minor amount of the dispersive medium such as water and crushed together in a ball mill. The resulting slime can be stirred for several days, adjusted for viscosity or moisture and defoamed in vacuo for improving its stability.
A plaster mold is assembled after the mold release agent is applied on the surfaces of the various mold components. The slime prepared in the above described manner is cast into the mold. With the absorption of moisture into the mold, a wall layer is formed on the mold surface and grown with the lapse of time.
For molding a hollow article, excess slime can be discharged when the wall thickness reaches a predetermined value.
Since the mold into which the slime has been cast as described above continues to absorb the moisture, water contents in the wall layer are decreased gradually so that the cast article is increased in hardness and contracted so as to be detached from the mold.
At this time, the cast article is removed and subjected to rough and finish processing steps, followed by drying. The plaster mold from which the cast article has been removed can be dried for repeated usage for molding.
A variety of alcohols can be used besides water as the dispersive medium used for the preparation of the starting slime of the powder mixture.
As a mold material, water permeative mold materials selected from the group of plastic materials superior in mechanical strength or wear resistancy and the metal-ceramic fiber composite materials can be used in place of plaster.
There is also known a method according to which a core is inserted and the powdered material is tamped, after which the core is removed to produce the mold of compacted powders to be used for molding. The mold is destroyed after termination of molding while the compacted powders are recycled into the batch of the powdered material. In this case, the powdered material can be advantageously reused.
The major problem in the casting-molding of the metallic and ceramic powders is presented in the drying process. There are two main steps in the drying process. In the first constant-rate drying step, the water is lost from the surface of the cast article, which then undergoes shrinkage in an amount corresponding to the volume of the lost water.
In the next reduced-rate drying step, the water is vaporized within the casting-molded product. The shrinkage which the molded product undergoes during this step is nil or at most negligible.
With the cast article with variable thicknesses, it takes some additional time until the portion of the cast article with a larger thickness shifts from the constant rate drying to the reduced rate drying, so that shrinkage does not proceed smoothly. In the case of a molded article with a larger thickness, it takes some additional time until the inner part of the article is dried, with the shrinkage on the superficial portions taking place more promptly than that at said inner portion. In this case, the molded article is likely to undergo strain or crevice formation in the course of the drying process.
In order to prevent strain or cracks from occurring, it is necessary that the inner part of the molded product in its entirety be cooled uniformly by using a lower drying temperature. To this effect, natural drying is possibly most preferred. However, natural drying is influenced by climatic conditions and need be carried out over an extended time while also requiring a large floor space and a lot of man-power.
The drying with the aid of hot air is also inconvenient in that the air volume and velocity, temperature and humidity need be maintained at uniform values, while it is also difficult to achieve a proper control because one has to take too many setting elements into account.
There is also known a high frequency drying method, according to which drying proceeds relatively uniformly. It is however difficult to dry the molded product without producing heat stresses in the respective portions as well as the inner and outer layers of the molded product. Thus, sporadic drying, crevices, cracks or strain is caused to a more or less degree with the result that the yield of the dried product is necessarily lowered.